Reading through how BMO explains it, it seems like pulling money out just adds back to the principle of my mortgage at the same mortgage interest rate, so no extra borrowing expense:
From BMO: “The re-borrowed funds are added to your mortgage principal at your existing interest rate for the remainder of the term.”
Only the pre payment money is available to withdraw. I can prepay up to 20% of my mortgage every year. This feature of withdrawing your prepayment is available on regular BMO mortgages, I just switched to BMO in a 3yr fixed because they gave me the best rate.
My personal banking is with simplii and I’m not impressed with their HISA so thought about this scheme as a replacement.